The present invention relates generally to tri-nickel aluminide materials of substantial strength and ductility. More specifically, it relates to compositions having a tri-nickel aluminide base and having substituents which impart to the base material a desirable combination of properties for use in structural applications.
It is known that polycrystalline tri-nickel aluminide castings exhibit properties of extreme brittleness, low strength and poor ductility at room temperature.
The single crystal tri-nickel aluminide in certain orientations does display a favorable combination of properties at room temperature including significant ductility. However, the polycrystalline material which is conventionally formed by known processes does not display the desirable properties of the single crystal material and, although potentially useful as a high temperature structural material, has not found extensive use in this application because of the poor properties of the material at room temperature.
It is known that tri-nickel aluminide has good physical properties at temperatures above 1000.degree. F. and could be employed, for example, in jet engines as component parts at operating or higher temperatures. However, if the material does not have favorable properties at room temperature and below the part formed of the aluminide may break when subjected to stress at the lower temperatures at which the part would be maintained prior to starting the engine and prior to operating the engine at the higher temperatures.
Alloys having a tri-nickel aluminide base are among the group of alloys known as heat-resisting alloys or superalloys. These alloys are intended for very high temperature service where relatively high stresses such as tensile, thermal, vibratory and shock stresses are encountered and where oxidation resistance is frequently required.
Accordingly, what has been sought in the field of superalloys is an alloy composition which displays favorable stress resistant properties not only at the elevated temperatures at which it may be used, as for example in a jet engine, but also a practical and desirable and useful set of properties at the lower temperatures to which the engine is subjected in storage and mounting and starting operations. For example, it is well known that an engine may be subjected to severe subfreezing temperatures while standing on an airfield or runway prior to starting the engine.
Significant efforts have been made toward producing a tri-nickel aluminide and similar superalloys which may be useful over such a wide range of temperature and adapted to withstand the stress to which the articles made from the material may be subjected in normal operations over such a wide range of temperatures.
For example, U.S. Pat. No. 4,478,791, assigned to the same assignee as the subject application, teaches a method by which a significant measure of ductility can be imparted to a tri-nickel aluminide base metal at room temperature to overcome the brittleness of this material.
Also, copending applications of the same inventors as the subject application, Ser. Nos. 647,326; 647,327; 647,328; 646,877 and 646,879, filed Sept. 4, 1984 teach methods by which the composition and methods of the U.S. Pat. No. 4,478,791 may be further improved. These applications are incorporated herein by reference.
We have now discovered a beneficial effect of carbon on tri-nickel aluminides.
The effect of carbon in Ni.sub.3 Al was previously studied by R. W. Guard and J. H. Westbrook (Trans. Met. Soc. AIME, Vol. 215, 1959, pp. 807-814). A hardness of .about.200 kg/mm.sup.2 was measured at room temperature for Ni.sub.3 Al containing 0, 0.2 and 2.0 atomic percent carbon, showing little carbon effect on the mechanical behavior of Ni.sub.3 Al. The solubility of carbon in Ni.sub.3 Al was determined to be 5.8 atomic percent (L. J. Huetter and H. H. Stadelmaier, Acta Met., Vol. 6, 1958, pp. 367-370). The solubility was extended to about 7.8 atomic percent by rapid solidification (K. H. Han and W. K. Choo, Scripta Met., Vol. 17, 1983, pp. 21-284). The above two papers did not deal with mechanical behavior.
Recently, iron base alloys in the Fe-Ni-Al-C system were investigated (A. Inoue, Y. Kojima, T. Minemura and T. Masumoto, Met. Trans. A, Vol. 12A, 1981, pp. 1245-1253). It was found that, by rapid solidification, nonequilibrium Ll.sub.2 phase alloys could be produced in this iron-base system in the composition range of 7-55 weight percent Ni, 8-9 weight percent Al and 0.8-2.4 weight percent C, the balance being iron. This nonequilibrium phase was found to be ductile by tensile tests. The yield strength increased with carbon concentration, from .about.900 MPa at 1.2 weight percent C to .about.1700 MPa at 2.4 weight percent C, in a matrix of Fe-20Ni-8Al. However, tempering the material at a temperature as low as 500.degree. C. for 1 hour resulted in the alloy becoming brittle due to phase decomposition. No further properties were reported for the embrittled material. The iron base material has no useful structural applications because of its tendency to return to an equilibrium condition and to acquire brittle properties over a period of time. High temperature use of the material accelerates its return to a brittle condition.